Devour X, where X is the number of creatures devoured this way (As this enters the battlefield, you may sacrifice any number of creatures. This creature enters the battlefield with X +1/+1 counters on it for each of those creatures.)

For example, if Thromok devours one creature, it will enter the battlefield with one +1/+1 counter on it. If it devours two creatures, it will enter with two +1/+1 counters for each of them, for a total of four +1/+1 counters. Devouring three creatures will produce nine +1/+1 counters, and so on.